Variable inductor



H. L. COBB Nov. 22, 1938.

VARIABLE INDUCTOR Filed Feb. 16, 1934 4 Sheets-Sheet 1 Nov. 22, 1938. H. L. COBB 2,137,392

VARIABLE INDUCTOR Filed Feb. 16, 1934 4 Sheds-Sheet 2 FEE-"6- Nov. 22, 1938. H, L, COBB 2,137,392

VARIABLE INDUCTOR Filed Feb. 16, 1934 4 Sheets-Sheet s NOV, 22, 1938. H, B 2,137,392

VARIABLE INDUCTO R Filed Feb. 16, 1954 4 Sheets-Sheet 4 m w i k B P Patented Nov. 22, 1938 VARIABLE KNDUCTOB Howard L. Gobb,

mesne assignments, to

America, New York, Delaware Boonton, N. 3., designer, by

Radio Corporation oi N. K, o corporation ol! Application February 16, 1934, Serial No. 311,811

1'7 Claims.

This invention relates to variable inductors for use at radio frequencies end particularly to inductors which have desirable electrical chorecteristics and simple mechanical constructions.

It has been proposed to vary the inductance of c. solenoidel winding by the use of cylindrical magnetic cores but variable inductors of this type are of relatively lerge size. The mounting of the coils or the core for longitudinal displacement presents dimcult mechanical problems, particulurly when 9. plurality of circuits are to be tuned simultaneously by a single control device.

An object of the invention is to provide variehie inductors in the form of a. spiral winding or windings and fist pieces of magnetic core meteriel, the total inductance being varied by relative mechanical displacement oi the core and the winding or windings. Other objects are to provide inductors for use at radio frequencies which are oi small dimensions and have fields of minimum volume, the inductors and their associated cores being of such. design that the some mechanicel and electrical characteristics may be readily imparted to a plurality of variable in ductors, whereby gauging oi the inductors for multiple tuned radio circuits is facilitated. A further object is to provide spiral. inductors, either single inductors or transformers, of small dimensions and of rugged mechanical design. A further object is to provide a variable inductor whose inductance may be varied over a relatively wide range of frequencies.

These and other objects and advantages of the invention will be apparent from the following specification when taken with the accompenying drawings, in which:

Figs. 1 and 2 are perspective views of variable inductors embodying the invention;

Fig. 3 is a circuit diagram of a. pair of cascaded tubes, the coupling system being a transformer embodying the invention;

Figs. 4. end 5 are somewhat diagrammatic views of different types of inductors in which two windlugs and two magnetic material bodies are employed;

Fig. ii is on elevation of a frame in which a. spiral coil may be mounted;

Fig. '7 is an elevation, with parts broken away, of o. coil and frame assembly;

Figs. 8 and 9 are side and edge elevations, respectively, of a completed assembly;

Fig. Iii is a. top view "of a. mold in which the coil, shown diagrammatically may be embedded in a. plastic molding composition;

Figs. 11 and 12 are side and edge elevations, re-

(Gl. till-2 12i spectively, of an assembly such so mode with the mold of Fig. lit;

Fig. 1c is s top view oi soother form oi mold;

Fig. it is c. section, es teiseu on line i i-id of Fig. 13, oi the mold assembly with c. coil in place therein;

Fig. Moi is c. fragmentary section of Fig. 13;

Figs. 15 end to are side and edge elevetions, respectively, of coil ossemblies produced by the mold oi Figs. 13 and 3. 2;

Fig. 17 is an elevation, of s transformer type of coil assembly;

Figs. 13 end is ere end views oi modified er rencements oi the megnetic body and coil elements oi. the inductor;

Fig. 20 is c perspective view oi the coil oss'em bly of Fig. iii;

Fig. 21 is u perspective view of one of the magnetic elements;

Fig. 22 is an end view and Fig. 23 is a central section of on inductor in which the magnetic elements ere moved cruelly; l

Figs. 24 end 25 are similer views oi smother arrangement for obtaining the cruel motion of the magnetic bodies;

26 is 9. side view of e. shielded variable inductor, one well of the shield being broken away; and

Fig. 27 is e. plan view of the some.

The common practice in tuning radio frequency circuits is to use fixed inductors and vary the capacitance by means of variable condensers. These circuits can also be tunedby using fixed condensers and varying the lnductences. If bodies of magnetic material of higher permeability than air are moved into the field of an inductor in such a manner as to embrace more or less of the field oi the inductor, the inductance of the sold inductor can be varied so that in a circuit containing 9. fixed condenser and such a. variable inductor a band of frequencies can be covered comparable to that of the fixed inductor and variable capacitor scheme.

The art of inductive tuning of radio frequency circuits, the manufacture of suitable magnetic meterlal and the theoretical considerations concerning the. some have been covered in the literature and this invention relates to special forms of devices for such inductive tuning.

One advantage of using fixed condensers and variable inductors with magnetic bodies tor tum ing radio frequency circuits over the common variable condenser method is that the selectivity is kept more nearly constant over the band covon line Me ered. In the. case of condenser tuning the resistance of the circuit increases nearly as the square of the frequency and since the inductance is constant the selectivity will be considerably lower at the high frequencies than at the low. With inductance tuned circuits, on the other hand, which are tuned by magnetic bodies, the resistance R, and inductance L, change together and in the same direction with changes in frequency so that the ratio of and therefore the selectivity are maintained practically constant.

Another advantage of the variable inductor scheme using magnetic bodies over the variable capacitor system is the reduction in space required for the component parts and therefore the possible decrease in cost of manufacturing. A particular advantage in this respect comes in the use of fiat spiral coils, which when properly constructed and associated with the other essential parts of the tuned circuit, can be made to occupy a comparatively small space. Other technical advantages of this combination are that the tuning curves can be made to follow a straight line frequency law, the gain over the broadcast band can be kept nearly constant, and in tuning over several bands of frequencies (for example the broadcast band and one or two of the short wave bands) several sets of capacitors can be arranged with a gang selector switch to accomplish the transfer in a simple manner.

In the simple type of variable inductor shown in Fig. 1, the winding or coil L is embedded in an insulating support i which is provided with an extension 2 for mounting the coil upon a fixed support. The coil L is a single layer spiral and the support i has the general form of a thin fiat disc. The magnetic body takes the form of two discs 3, 3 of magnetic material and a yoke 4 which extends along about one-half the circumference of the discs. The magnetic body is mounted on an arm 5 which is fixed to a. shaft 6, the arm preferably terminating in a weight 8. which balances the weight of the magnetic body and supporting arm. The inductance. of coil L is a minimum when the parts are in the positions shown and, by turning shaft 6 counterclockwise, the magnetic body may be moved to encompass more or less of the coil to increase its inductance.

The yoke 4 is particularly important as the magnetic circuit from one disc to the other must I be closed through magnetic material to keep the effective permeability as high as possible to secure a high ratio of maximum to minimum inductance. When made, as illustrated, of diiferent sections, the parts of the magnetic body may be rigidly connected to each other by cement, machine screws or rivets. The sectional construction presents certain advantages'in manufacturing but is not essential as the entire magnetic body may be molded in one piece or may be assembled from parts of other design. The magnetic body or magnetic body sections may be formed to predetermined size and shapeby the known methods of forming core material, by pressure molding, from mixtures of powdered magnetic material and reactive resins. The maximum inductance is also affected by the air gap between the discs 3 and, in practical commercial constructions, I have found that a clearance of from two to four thousandths of an inch at each side of the coil support is sumcient for good mechanical operation and does not appreciably cut down the maximum inductance. Clearances of this order call for a close dimensioning of the coil assembly, but this is practical when the coils are mounted by the methods which will be described.

As shown in Fig. 2, the magnetic body I, 4 may be fixed and the coil L pivotally supported on a bracket 5 of insulating material which is carried by the shaft 6'. The support I has the form of a'disc with three lugs or radial extensions 2', one extension being a guide which remains within the gap when the shaft is 1'0- tated to its extreme position corresponding to minimum inductance. The other lugs or ears 2' serve as means for securing the disc to the bracket 5' and as supports for the terminals of coil L. A flexible lead 9 is shown as connecting the high potential terminal to a fixed terminal i0, while a lead H grounds the other coil terminal. One advantage obtained by moving the coil is that the moving parts are very light and therefore the friction is very small, thus making it possible to use smaller partsfor the tuning mechanism.

A tuned transformer construction for coupling cascaded radio frequency amplifier tubes T is shown in Fig. 3. The mechanical construction is like that of Fig. l, but the stationary windings include the primary P and secondary S of a spirally wound transformer. A fixed condenser |2 is'shunted across the winding S to complete the tuned circuit which works into the second tube. The primary P is connected between the plate of the first tube and the plate battery B in the usual manner.

In the case of a multistage amplifier, all of the tuning elements may be secured to the same shaft. When an even number of circuits are tuned by moving the magnetic bodies, the several magnetic bodies may be arranged in two groups on opposite sides of the shaft to balance each other. When the moving coil system is employed the total weight is quite small and, in general, it will not be necessary to balance the moments about the shaft due to gravity. Shielding of the several inductors in such an amplifier can be accomplished in a manner similar to that used in condenser tuned multistage tuned circuits.

As shown in Fig. 4, two coils L, L mounted in the same plane and connected in series, and their total inductance is varied by rotating two magnetic bodies It that are mounted on a common shaft II. The magnetic bodies I3 are each of the disc and yoke design shown in the preceding views. A modified magnetic body design for double tuning is illustrated in Fig. 5. The ends of an elongated magnetic body I5 are slotted to encompass the coils and the magnetic circuit of each pair of opposed end sections is completed through the central portion of the magnetic body which thus acts as the yoke section of the separate magnetic bodies l3 of Fig. 4.

The advantage of the double tuning magnetic body construction is that it provides a perfect balancing without the use of counterweights. The same magnetic body construction may, of course, be used with two coils which form parts of two different tuned circuits. Further, any even number of circuits may be ganged together by mounting the tuning magnetic bodies of the several pairs of circuits on the same shaft. Suitable shielding can be provided between the inductors to prevent coupling.

One method of forming a spiral coil assembly is illustrated by Figs. 6 to 9, inclusive. A frame i6 is cut from thin cardboard, Bakelite or other insulating material, the outline of the frame being that oi an annulus provided with radial ears. The opening in the frame is such as to snugly receive a spiral coil L, and the thickness of the frame is approximately that of coil L. The fiat spiral coil is wound in a form and temporarily held together wlth pyroxylin cement. The coil is placed in frame 5 and terminals i7, it are cut from very thin metal, such as 0.0015 inch phosphor bronze, and placed on the frame. A thin strip of insulating material is arranged between the coil L and the portion of terminal 38 which extends to the center of the spiral winding. The ends of the coil are soldered to the terminals I], I8, and the frame is then closed by two thin cover sheets it which have the same outline as the outer edge of the frame i6 except that the ears are cut from one sheet to expose terminals I! and i8. One side of each sheet it is coated with a thin layer of a varnish prepared from a reactive synthetic resin, and the coating is allowed to dry before the sheets are applied to the frame. When the assembly is completed, it is pressed between two fiat plates and at the same time heated to cause the reactive resin to penetrate the voids and cure to a permanent state. Holes are then punched in the terminal ears and metal eyelets 20 inserted for convenience in making electrical and mechanical connections to the assembly.

Another practical method of construction is to embed the coil L in an insulating cover or disc 21 by molding a reactive resin binder upon the coil. In this process, the coil L with attached eyelet terminals 22 is placed in a lower mold 225, Fig. 10, the reactive binder in powder form is placed in the mold, and the mold is then closed and heated under pressure to convert the resin to its final hardened condition. Coils manufactured in this manner are perfectly flat and the assembly is but little thicker than the outside diameter of the wire.

A preferred method of assembly, as illustrated in Figs. 13 to 16, inclusive, comprises molding the winding between two sheets of paper 24 which are impregnated with a reactive resin. The lower mold section 25 has a recess for snugly receiving the lower paper sheet 24, which lower sheet is not provided with extensions for covering the termito serve as guides for accurately positioning the coil L within the mold. Some or all of the pins may serve as knock-out pins, and the upper mold section 29 is appropriately recessed to receive the upper ends of the pins.

The impregnated sheets 24 are perforated to receive the several pins and the sheets are preferably of very soft paper, such as blotting paper,

which is spongy or porous and may be indented by the winding and terminals during the mold-=- ing operation. When cured under heat and pressure, the reactive resin flows and penetrates the voids around the coil and the terminal strip 30 and insulating strip 3| which overlie the coil are pressed into the spongy paper, thus resulting in a solid block of insulating material, with smooth external surfaces, in which the coil and terminals are accurately located. When spongy or porous paper is used in the molding process, there will be nc'projections or thickened areas where the thin connecting strips pass over the coil. Eyelets 2'! are inserted in the holes in the terminal strip 2"! after the unit is removed from the mold.

As shown in Fig. 17, the same methods may be employed for securing the spiral transformer windings P, within a thin disc of insulating material. The disc has a projecting guide lug 32 and two sets of terminal lugs 33 and 34 for the respective windings.

A modified form of moving coil system as shown in Fig. 18, includes a semi-circular yoke 35 at the edge of the coil disc l' which is remote from the stationary magnetic body 3, 4. The remaining elements of the assembly may be substantially the same as shown in Fig. 2 and have been identified by the reference numerals of that view. lhe

advantage of this construction is that the coil is completely enclosed by the magnetic material when the coil is moved counter-clockwise into its final position corresponding to maximum inductance. The extra yoke 35 increases the effective permeability of the magnetic circuit and thereby increases the maximum inductance which may be obtained with a given inductor.

Another eflicient variable inductor, as shown in Figs. 19 to 21, includes a coil mounted within a disc or support at which has a transverse fln 3i carryingeyelets 38 to which the coil terminals are secured and which serve as means for mounting the coil. The magnetic material is formed as two substantially semi-circular discs 39 which are slotted to receive the coil disc, and the magnetic body members 39 are mounted on brackets 4b which are simultaneously adjusted, but in opposite directions, about a shaft M by the right and left hand screw at. The nuts $3 on the screw 42 are held snugly to the brackets Ml, but may rock on the brackets to permit the slight turning movement which takes place as the magnetic bodies are moved towards and away from each other.

Variable inductors of this type have the same advantage, as to higher maximum inductance, as the form shown in Fig. 18, and have additional advantages, such as, convenience in molding each magnetic body in one piece, increased efliciency since the efiective magnetic permeability will be somewhat greater as the magnetic path includes fewer air gaps at maximum inductance, and reduced,costs since the magnetic bodies may have smaller overall dimensions and are duplicates. In place of arranging the supporting fin at right angles to the plane of the coil, it may be located in the plane of the coil in which case the magnetic bodies must be slotted to accommodate the lets which are electrically connected to the coil.-

One magnetic body 41 has the form of a flat cup or flanged disc which is notched to receive the lugs 48 oil the coil disc, and the core 48 is an annular disc. The magnetic bodies are secured to threaded collars 49, 50, respectively, on the double-threaded screw 5! which is mounted in brackets 52. The magnetic body elements are provided with fingers or keys 53 which extend into a keyway 54 inthe support 45 to prevent rotation 01 the magnetic bodies when the screw is rotated by the knob 55. The ends or the screw are threaded in opposite directions and the magnetic bodies are therefore moved simultaneously but in opposite directions when the knob 55 is rotated.

The embodiment shown in Figs. 24 and 25 differs from the last described construction in that both movable magnetic bodies ll are annular discs and an annular magnetic body 41 which surrounds the periphery of the coil disc is fixed to the base by a support 56, the annular magnetic body being notched at one side to receive the lugs 45 of the coil disc. The discs 48 fit closely within the annular magnetic body when adjusted towards the coil disc to increase the inductance. The remaining elements may be as shown in Figs. 22 and 23, and are identified by the same reference numerals.

A practical unit assembly of a variable inductor for tuning a radio frequency circuit is shown in Fig. 26. struction is o! the type shown in Fig.18, and includes a coil disc 51 which is partially enclosed by a semi-circular core 58, these parts being carried by a bracket 59 which is secured to the tuning shaft 60. One terminal lug 5| is grounded on the bracket and chassis by a lead 61, and the other coil terminal 63 is connected'to a soldering lug 64 by a flexible lead". The soldering lug is mounted on an insulated block which is secured to the bent strap 61 on which the magnetic body assembly 58, '10 is mounted. The magnetic body assembly has the form shown in Fig. 1 and includes two spaced discs 58 between whichithe guide lug 59 extends even in the minimum inductance adjustment, and the fixed magnetic body yoke I0. An L-shaped bracket "H serves as a common support for the coil and magnetic body members, the core bracket 61 being secured to the vertical arm of the bracket H, while the horizontal arm is provided with a pair of integral ears or journals '12 in which the shaft 60 is mounted. A 'metal shield 13, in the form of a narrow box with one open side, may be slipped over the entire inductor assembly and retained by frictional engagement with the bracket 1i. Connection may be made to the high potential terminal lug 64 by a wire 14 which enters the shield through an insulating bushing 15. A condenser, not shown, is connected between lead 14 and the chassis or ground to complete the tuned circuit.

The following data is given as illustrative of an appropriate design fora variable inductor such as shown in Figs. 26 and 27. The coil disc 51 had a diameter of 1 inches and a thickness of 0.02 inch. The coil was a single layer of 36 turns, about 1% inches outside diameter, of Litzendraht cable consisting of ten strands of No. 41 enameled wire. The magnetic body sections 68 were inch thick and of 1 inches diameter, with a spacing or gap between the sections of about 0.028 inch. The copper shield can 13 was fl"x4%"x3". When the coil was shunted by a fixed condenser of appropriate value, the circuit was adapted for tuning over the broadcast range of from 500 to 1500 kilocycles.

A number of such units may be mounted side by side and operated by a common shaft for mul- The coil and magnetic body con:

tiple circuit tuning. As the shaft is turned clockwise, the coil enters between the magnetic body members 68 and the inductance is increased.

It will be understood that there is considerable latitude in the design'of the electrical and mechanical elements of the novel variable inductors. The illustrated embodiments are typical of appropriate designs but other forms may be used to obtain other mechanical movements or other electrical effects which are contemplated by my invention. For simplicity of illustration, the majority of coil structures are shown as single spiral windings but it is obvious that multiple windings may be employed whenever a transformer type of circuit is desired. Similarly, the magnetic bodies are all shown as of symmetrical shape and density, but the magnetic body shape may be altered to obtain different rates of inductance changes for similar adjustments of the coil and magnetic body. For example, if the initial rate of change of inductance on entering the coil is to be raised, the edge of the magnetic body at the point of entrance can be made denser by using more magnetic material per unit volume at this point than in the rest of the magnetic body, or the shape of the magnetic body at the entrance can be changed to embrace the coil's field at a more rapid rate.

Although the drawings illustrate inductors which may be continuously tuned over wide ranges, it is obvious that the invention is equally useful in the construction of inductors that may be varied over relatively small ranges and also to inductors that are not varied during the usual tuning adjustments of the circuit or transmission system in which the inductor is included.

The construction may be simplified in the case of intermediate frequency transformers or chokes where the coil and magnetic body are initially adjusted to one relatively fixed relationship which will be maintained throughout the operation of the receiver.

It is therefore apparent that various changes may be made in the supporting structure of the spiral coil or coils, in the design of the magnetic bodies, and in the mechanism .or supporting and adiusting the parts without departure from the spirit of my invention asset forth in the following claims.

I claim:

1. A variable inductor comprising a fiat spiral winding, a body of magnetic material, and means mounting said winding and magnetic material body for relative movement, thereby to vary the inductance of said winding.

2. A variable inductor as claimed in claim 1. wherein said mounting means includes a flat disc of insulating material in which said winding is embedded.

3. A variable inductor as claimed in claim 1, wherein said mounting means includes a support for said winding, and means for adjusting said support and winding towards and away from said magnetic body.

4. A variable inductor comprising a flat spiral coil and means providing a rigid insulating support for the same, a magnetic body, and means for eflecting relative displacement of said coil and magnetic body; said magnetic body comprising a pair of fiat sections spaced apart to receive said coil and supporting means, and a yoke section connecting said fiat sections.

5. A variable inductor as claimed in claim 4, wherein said rigid supporting means comprises a disc molded on and enclosing said coil, and

mamas lugs projecting radially from the. disc, incombination with terminals for said coil secured to certain of said lugs.

. 6. A variable inductor comprising a coil supporting disc in which a coil is embedded, a magnetic body including a plurality of relatively movable sections, at least one of said sections being slotted in the plane of said disc, and means for effecting relative movement of said disc and magnetic body to vary the inductance of said coil.

' '1. A variable inductor as claimed in claim 6, wherein one 01 said magnetic body sections is fixed with respect to said coil supporting disc.

8. A variable inductor as claimed in claim 6, wherein said disc is stationary,- and said last means moves said magnetic body sections simultaneously and in opposite directions.

9. A variable inductor assembly comprising a support, a flat disc in which a coil is embedded, lugs projecting from said disc and carrying terminals for said coil, a magnetic body including disc sections at opposite sides of the plane of said disc and a yoke section for completing a magnetic circuit including said disc sections, means mounting said coil disc and magnetic body on said support .for relative movement, thereby to vary the inductance on said coil, and a shield for said coil and magnetic body.

10. A variable inductor assembly as claimed in claim 9, in combination with a magnetic yoke section fixed with respect to said coil disc. I

11. A variable inductor assembly as claimed in claim 9, wherein said mounting means comprises means securing said magnetic body to said sup-- port, and means pivotally mounting said coil disc on said support.

'12. A variable inductor comprising a coil unit including a flat disc of insulating material enclosing a spiral coil and having a plurality of radial lugs, means including connections to a plurality of said lugs providing a stationary support,

for said coil unit, a magnetic body including a section at each side of said disc, and tuning means for moving said magnetic body sections simultaneously and in opposite directions to varythe inductance of said poll.

13. A variable inductor as claimed in claim 12, wherein said tuning means includes a double threaded screw for moving said magnetic body sections axially of said coil.

14. A variable inductor as claimed in claim 12, wherein one of said magnetic body sections has a flange for extending over the edge of said flat disc when said magnetic body sections approach the disc to increase the inductance of said coil.

15. A variable inductor as claimed in claim 12, wherein said magnetic body sections are of circular disc form, in combination with a stationary annular ring or magnetic material surrounding the edge of said flat disc.

16. Apparatus of the type stated comprising two spiral coils, a magnetic body for cooperation with each coil, and means mounting said coils and magnetic bodies for relative movement to effect simultaneous variations of the inductances of said coils.

17. A variable inductor comprising a flat spiral 'winding having an inner end and an outer end,

a terminal strip having an inner end conductively connected to the inner end of the. winding, said terminal strip extending radially beyond said winding, a sheet of insulating material between the winding and the terminal strip, a terminal secured to the outer end oi the winding, a rigid disc of insulating material enclosing said coil and having radial projections providin supports for the terminals, a body of magnetic material including auction at each side of said disc and means for moving said magnetic body sections simultaneously and in opposite directions to vary the inductance of said coil.

, HOWARD L. COBB. 

